Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

spec/async_spec.rb

@@ -1,8 +1,4 @@
 context 'with asynchronous suggestions enabled' do
-  before do
-    skip 'Async mode not supported below v5.0.8' if session.zsh_version < Gem::Version.new('5.0.8')
-  end
-
   let(:options) { ["ZSH_AUTOSUGGEST_USE_ASYNC="] }
 
   describe '`up-line-or-beginning-search`' do
@@ -30,55 +26,6 @@ context 'with asynchronous suggestions enabled' do
       end
     end
   end
-
-  it 'should not add extra carriage returns before newlines' do
-    session.
-      send_string('echo "').
-      send_keys('escape').
-      send_keys('enter').
-      send_string('"').
-      send_keys('enter')
-
-    session.clear_screen
-
-    session.send_string('echo')
-    wait_for { session.content }.to eq("echo \"\n\"")
-  end
-
-  it 'should treat carriage returns and newlines as separate characters' do
-    session.
-      send_string('echo "').
-      send_keys('C-v').
-      send_keys('enter').
-      send_string('foo"').
-      send_keys('enter')
-
-    session.
-      send_string('echo "').
-      send_keys('control').
-      send_keys('enter').
-      send_string('bar"').
-      send_keys('enter')
-
-    session.clear_screen
-
-    session.
-      send_string('echo "').
-      send_keys('C-v').
-      send_keys('enter')
-
-    wait_for { session.content }.to eq('echo "^Mfoo"')
-  end
-
-  describe 'exiting a subshell' do
-    it 'should not cause error messages to be printed' do
-      session.run_command('$(exit)')
-
-      sleep 1
-
-      expect(session.content).to eq('$(exit)')
-    end
-  end
 end